Effect of temperature on biogas yield increase and formation of refractory COD during thermal hydrolysis of waste activated sludge

Abstract

Thermal hydrolysis (TH) increases the anaerobic biodegradability of waste activated sludge (WAS), but also refractory organic and nutrient return load to a wastewater treatment plant (WWTP). This could lead to an increase in effluent chemical oxygen demand (COD) of the WWTP. The aim of this study was to investigate the trade-off between increase in biogas production through TH and anaerobic digestion and increase in refractory COD in dewatered sludge liquors at different temperatures of TH in lab-scale. WAS was thermally hydrolyzed in temperature range of 130-170 °C for 30 min to determine its biomethane potential (BMP). After BMP test, sludge was dewatered and sludge liquor was aerated in Zahn-Wellens test to determine its non-biodegradable soluble COD known as refractory soluble COD (sCOD_ref). With increasing temperature in the range of 130-170 °C, BMP of WAS increased by 17-27%, while sCOD_ref increased by 3.9-8.4%. Dewaterability was also enhanced through relative increase in cake solids by 12-30%. A conversion factor was defined through mass balance to relate sCOD_ref to volatile solids of raw WAS. Based on the conversion factor, expected increase in effluent CODs of six WWTPs in Berlin were predicted to be in the range of 2-15 mg/L after implementation of TH at different temperatures. It was concluded that with a slight decrease in temperature, formation of sCOD_ref could be significantly reduced, while still benefiting from a substantial increase in biogas production and dewaterability improvement.

Keywords: Anaerobic digestion; Biomethane potential; Effluent COD; Sludge liquor; Thermal hydrolysis; Zahn-Wellens test.